Semiconducting　open-shell　radicals　(SORs)　have　promising　potential　for　the　development　of　phototheranostic　agents,　enabling　tumor　bioimaging　and　boosting　tumorous　reactive　oxygen　species　(ROS).　Herein,　a　new　class　of　semiconducting　perylene　diimide　(PDI),　designated　as　PDI(Br)n　with　various　numbers　of　bromine　(Br)　atoms　modified　on　PDI＇s　bay/ortho　positions　is　reported.　PDI(Br)n　is　demonstrated　to　transform　into　a　radical　anion,　[PDI(Br)n]•−,　in　a　reducing　solution,　with　a　typical　g-value　of　2.0022.　Specifically,　[PDI(Br)4/6]•−　is　generated　in　the　weakly　reductive　tumor-mimicking　solution　and　exhibits　high　stability　in　air.　Quantum　chemical　kinetic　simulation　and　ultrafast　femtosecond　transient　absorption　spectroscopy　indicate　that　[PDI(Br)6]•−　has　a　low　π–π　stacking　energy　(0.35 eV),　a　fast　electron　transfer　rate　(192.4 ps)　and　energy　gap　of　PDI(Br)6　(ΔES1,　T1= 1.307 eV,　ΔES1,　T2= 0.324 eV)　respectively,　which　together　result　in　excited-state　charge　transfer　characters.　The　PDI(Br)6　nanoparticle　radicals,　[PDI(Br)6]　NPs•−,　specifically　enable　chemodynamic　and　type-I　photodynamic　ROS　generation　in　tumors,　including　superoxide　and　hydroxyl　radicals,　which　elicit　immunogenic　cell　death　effect.　Also,　[PDI(Br)6]　NPs•−　facilitate　activatable　bioimaging-guided　therapy　due　to　their　photoacoustic　signal　at　808　nm　and　NIR-II　emission　at　1115 nm.　The　work　paves　the　way　for　the　design　of　SORs　for　precise　cancer　theranostics.　©　2025　The　Author(s).　Advanced　Science　published　by　Wiley-VCH　GmbH.